Researchers with Symantec have uncovered additional clues that point to Chinese hacker involvement in attacks against a large number of Western companies, including major U.S. defense contractors.

The attacks use malicious PDF documents that exploit an Adobe Reader bug patched last month to infect Windows PCs with "Sykipot," a general-purpose backdoor Trojan horse.

According to findings published Thursday by Symantec's research team, a "staging server" used by the attackers is based in the Beijing area, and is hosted by one of the country's largest Internet service providers, or ISPs.

The staging server stores new files, many of them malformed PDFs, that are used to infected machines. Symantec found more than 100 malicious files on the server; many had been used in Sykipot campaigns.

Researchers also said that one of the attackers who connected to the staging server did so from Zhejiang province on China's eastern coast. Hangzhou is that province's capital and largest city.

Previously, Symantec had confirmed that the Sykipot attacks had been aimed at people working at major defense contractors, and at a smaller number of individuals employed in the telecommunications, manufacturing, computer hardware and chemical sectors.

Lockheed Martin, whose security team was among those who reported the Reader vulnerability to Adobe, may have been one of the targeted defense contractors.

After digging through the staging server, Symantec found clues that led it to a second system where the same group hosted a tool that automatically modifies files, again including PDFs, as part of its strategy to evade detection by antivirus software.

Like other authors of targeted attacks, the Sykipot gang tags each campaign with an identification number so that it can evaluate each assault's effectiveness. The unique identifiers are hard-coded into the malware, said Symantec.

Adobe began patching the Reader vulnerability exploited by the Sykipot attacks on Dec. 16, and wrapped up the fixes on Jan. 10.

Although Symantec did not come out and name China as the home base of the Sykipot hackers, it came close.

"The attackers are familiar with the Chinese language and are using computer resources in China," the company said. "They are clearly a group of attackers who are constantly modifying their creation to utilize new vulnerabilities and to evade security products and we expect that they will continue their attacks in the future."